Rotational stiffness of deeply embedded column-base connections

Deeply embedded column-base (DECB) connections comprise a steel column embedded into a concrete footing, and are common in mid-rise to high-rise steel moment resisting frames. These are often idealized as fixed or pinned in seismic simulations because validated models are not available to estimate their true stiffness; this results in highly erroneous simulations of structural response. A method to characterize the rotational stiffness of deeply embedded column-base connections is presented. The method reflects physical insights developed during a recent experimental program on DECB connections, while also retaining a degree of simplicity for application in practical settings. The method considers deformations of several components within the connection, including bending and shear deformations of the embedded column, in addition to bearing deformations of the concrete adjoining the embedment. To resolve the internal indeterminacies of stress distribution within the connection, the method relies on an adaptation of the Winkler beam theory, supplemented by empirical assumptions. The results are examined against nine full-scale experiments from two test programs, of which one focuses on DECB connections (with embedments in the range of 508-762 mm), whereas the other focuses on shallowly embedded column-base connections (embedments 203-406 mm). The method is reasonably accurate for all the test specimens, with an average test-predicted ratio of 1.20 with a coefficient of variation of 0.21. Limitations of the proposed approach are discussed.